WASHINGTON, April 9 — The three-star Marine Corps general who was the military's top operations officer before the invasion of Iraq expressed regret, in an essay published Sunday, that he did not more energetically question those who had ordered the nation to war. He also urged active-duty officers to speak out now if they had doubts about the war.

Lt. Gen. Gregory Newbold, who retired in late 2002, also called for replacing Defense Secretary Donald H. Rumsfeld and "many others unwilling to fundamentally change their approach." He is the third retired senior officer in recent weeks to demand that Mr. Rumsfeld step down.

In the essay, in this week's issue of Time magazine, General Newbold wrote, "I now regret that I did not more openly challenge those who were determined to invade a country whose actions were peripheral to the real threat — Al Qaeda."

The decision to invade Iraq, he wrote, "was done with a casualness and swagger that are the special province of those who have never had to execute these missions — or bury the results."

Though some active-duty officers will say in private that they disagree with Mr. Rumsfeld's handling of Iraq, none have spoken out publicly. They attribute their silence to respect for civilian control of the military, as set in the Constitution — but some also say they know it would be professional suicide to speak up.

"The officer corps is willing to sacrifice their lives for their country, but not their careers," said one combat veteran who says the Pentagon's civilian leadership made serious mistakes in Iraq, but has declined to voice his concerns for attribution.

Many officers who served in Iraq also say privately that regardless of flawed war planning or early mistakes by civilian and military officers, the American public would hold the current officer corps responsible for failure in Iraq. These officers do not want to discuss doubts about the mission publicly now. General Newbold acknowledged these issues, saying he decided to go public only after "the encouragement of some still in positions of military leadership" and in order to "offer a challenge to those still in uniform."

A leader's responsibility "is to give voice to those who can't — or don't have the opportunity to — speak," General Newbold wrote. "Enlisted members of the armed forces swear their oath to those appointed over them; an officer swears an oath not to a person but to the Constitution. The distinction is important."

General Newbold served as director of operations of the Joint Chiefs of Staff from 2000 through the Sept. 11, 2001, attacks and the war in Afghanistan. He left military service in late 2002, as the Defense Department was deep into planning for the March 2003 invasion of Iraq.

"I retired from the military four months before the invasion, in part because of my opposition to those who had used 9/11's tragedy to hijack our security policy," General Newbold wrote.

His generation of officers thought it had learned from Vietnam that "we must never again stand by quietly while those ignorant of and casual about war lead us into another one and then mismanage the conduct of it," General Newbold wrote.

The "consequence of the military's quiescence" in the current environment, he wrote, "was that a fundamentally flawed plan was executed for an invented war, while pursuing the real enemy, Al Qaeda, became a secondary effort."

A senior Pentagon official on Mr. Rumsfeld's staff said Sunday that the Pentagon leadership provided ample opportunity for senior officers to voice concerns.

"It is hard for the secretary and the rest of the policy leadership to understand the situation if they are not getting good, unvarnished advice from military commanders," the civilian official said.

While General Newbold said he did not accept the rationale for invading Iraq, he wrote that "a precipitous withdrawal would be a mistake" because it would tell the nation's adversaries that "America can be defeated, and thus increase the chances of future conflicts."

General Newbold's essay follows one on March 19, by another retired officer, Maj. Gen. Paul D. Eaton, who commanded the training of Iraqi security forces in the year after Baghdad fell. General Eaton wrote an Op-Ed article in The New York Times criticizing Mr. Rumsfeld's management of the war, adding, "President Bush should accept the offer to resign that Mr. Rumsfeld says he has tendered more than once."

When asked about that essay, President Bush rejected the call to dismiss Mr. Rumsfeld, repeating as he often has that he was satisfied with Mr. Rumsfeld's performance.

On April 2, Gen. Anthony C. Zinni, who previously led the military's Central Command, responsible for operations in the Middle East, said in a television interview that Mr. Rumsfeld, among others, should be held accountable for mistakes in Iraq and that he should step down.

General Newbold has been quoted previously describing his concerns about Iraq planning, including in "Cobra II," a book by Michael R. Gordon, chief military correspondent for The New York Times, and Bernard E. Trainor, a retired Marine lieutenant general who is a former military correspondent for the newspaper. In the book General Newbold is described telling fellow officers that he considered the focus on Iraq to be a strategic blunder and a distraction from the real counterterror effort. He is also quoted as expressing concern about Mr. Rumsfeld's influence on war planning, in particular his emphasis on assigning fewer troops to the invasion.

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